1. Field of the Invention
The subject matter of this invention resides in the field of ceramics, particularly in the area of forming methods for producing shaped ceramic articles.
2. Description of the Prior Art
The fabrication of shaped ceramic articles generally requires as a first step the formation of a green body, i.e. the molding of the mineral raw materials into a shape corresponding substantially to that of the ultimately desired ceramic article. The green body is then dried and fired to a hard, permanent shape. Many techniques known in the art can be employed to produce ceramic green bodies. A number of these, for example, powder pressing, extrusion, plastic forming and slip casting are described by Kingery (Introduction to Ceramics, John Wiley & Sons, Inc., New York, 1960, pp. 33-77) and Norton (Elements of Ceramics, Second Edition, Addison-Wesley Publishing Company, Reading, Mass., 1974, pp. 92-153).
Slip casting, a well-known and widely-used technique, is also described by Norton (Fine Ceramics, Technology and Applications, McGraw-Hill, Inc., New York, 1970, pp. 101-129). This method involves pouring a suspension or slip into a porous plaster mold. As the mold adsorbs water from the suspension a hard layer of clay is built up. The process is continued until the interior of the mold is filled. The molded clay body is then dried and fired.
It is well recognized in the ceramics art that among the most important properties required in a casting slip are low drying shrinkage and good flow properties, that is, the slip must have as low a water content as possible so as to minimize shrinkage on drying and yet must contain sufficient water to flow readily. These conflicting requirements are usually met by employing deflocculating agents which permit the formation of a fluid suspension with a relatively small amount of water. Thus, conventional casting slips ordinarily contain no more than about 15-25 percent water and on drying undergo a volume shrinkage of about 5-10 percent and no more than about 15-20 percent.
Other molding techniques have also been described. Thus, the E. G. Foster, et al. U.S. Pat. No. 3,526,685 filed Sept. 22, 1967, issued Sept. 1, 1970 discloses a process for producing foamed gypsum castings which comprises casting a foaming slurry of calcium sulfate in a mold having its internal faces lined with non-porous polyolefin-coated material.
Much current ceramic research has focused on producing a high-strength calcium phosphate ceramic, for example hydroxylapatite or whitlockite, in a form suitable for use as a surgical prosthetic device. This effort has primarily centered around powder pressing methods and has produced porous materials generally lacking the strength required in many implant devices.
Recently, Jarcho (U.S. Pat. No. 4,097,935, filed Jan. 31, 1977, issued July 4, 1978) has described a process for producing a novel, high-strength, non-porous ceramic form of hydroxylapatite which involves the precipitation of hydroxylapatite from aqueous solution in a gelatinous state, drying the gelatinous material and then sintering to produce the novel ceramic. However, the gelatinous precipitate which contains large amounts of occluded water undergoes substantial shrinkage upon drying resulting in separation, cracking or fracture, and conventional molding techniques have not been completely satisfactory when it is desired to produce large, defect-free, shaped bodies. Thus, there is need for a simple, economical and reproducible forming process for producing defect-free, shaped green bodies of hydroxylapatite and whitlockite which can be used to overcome this problem.